The present invention relates to a color cathode ray tube to be used for e.g. a display for a television broadcast receiver (hereinafter referred to as a television) or a computer, and a glass panel to be used for such a cathode ray tube.
Firstly, the construction of a color cathode ray tube will be described referring to the attached drawings. FIG. 1 is a partially cross-sectional view of the entirety of the cathode ray tube.
The envelope of the cathode ray tube 1 is constituted by a glass bulb 2 which basically comprises a panel 3 for displaying picture images, a funnel-shaped funnel 4 sealingly bonded to the panel 3 and a neck 5 accommodating an electron gun 17. The panel 3 is constituted by an approximately rectangular face portion 7 constituting a picture image-displaying screen and a skirt portion 6 extending in a direction substantially perpendicular to the face portion 7 from its periphery via a blend R portion 11.
An explosion proof reinforcing band 8 is wound around the circumference of the skirt portion 6 to maintain the panel strength and to prevent scattering upon breakage. On the inner surface side of the face portion 7, a phosphor screen 12 which emits fluorescence by electron beam bombardment from an electron gun 17 and an aluminum film 13 to reflect the fluorescence emitted from the phosphor screen 12 towards the rear side of the cathode ray tube (towards the funnel 4 side), to the front side (to the face 7 side), are laminated, and a shadow mask 14 which regulates the position for electron beam bombardment, is further provided. The shadow mask 14 is fixed to the inner surface of the skirt portion 6 by stud pins 15. Further, A in FIG. 1 indicates a tube axis connecting the center axis of the neck 5 and the center axis of the panel 3.
Such a panel 3 is sealingly bonded to a seal edge portion of the funnel 4 by a sealing material such as a solder glass provided at the seal edge portion corresponding to the end portion of the skirt portion 6, whereby a sealing portion 10 is formed.
The glass bulb for a color cathode ray tube having the above construction, is used as a vacuum vessel, whereby atmospheric pressure is exerted to the outer surface, and consequently, a tensile stress attributable to an asymmetrical shape as is different from a spherical shell, is exerted over a relatively wide range (a stress formed by a difference between the inner and outer pressures when the glass bulb is evacuated, will hereinafter be referred to as a vacuum stress).
Further, in the process for producing a cathode ray tube, especially when evacuation is carried out while maintaining the bulb at a high temperature of about 350xc2x0 C., a stress will be formed by the temperature difference resulting in such a step (the stress formed in such a heating step will be hereinafter referred to as a thermal stress) and will add to the above-mentioned vacuum stress, whereby in an extreme case, vigorous implosion may occur by instantaneous inflow of air and its counteraction, and thus damages may extend to the surrounding.
In the case of cathode ray tubes of other types such as projection type cathode ray tubes (projection tubes) or black and white (monochrome) cathode ray tubes, it is unnecessary to attach various parts such as a shadow mask to the inside of the panel, as mentioned above. Whereas, in the case of a color cathode ray tube, it is likely that the inner surface of the panel is damaged in a step of attaching such parts.
To guarantee prevention of such problems, an external pressure loading test is carried out by pressurizing a glass bulb uniformly scratched by 3150 emery paper, by air pressure or hydraulic pressure, taking into consideration the practical useful life of the cathode ray tube and the intensity of scratching on the glass surface which takes place in the step of assembling the glass bulb and the cathode ray tube, whereby the difference between the inner and outer pressures upon breakage, is determined, and the glass bulb is made to be durable against at least 0.3 MPa of such a pressure difference.
Further, in recent years, televisions are desired to have a large screen and a flat face portion and to have the weight reduced. If the wall thickness is simply made thin to accomplish such weight reduction, the above-mentioned vacuum stress will increase. Therefore, it is necessary to improve the strength of the panel, and some strengthening methods have been developed for this purpose.
Heretofore, as a means to reduce the weight of the glass bulb for a cathode ray tube, it has been practically proposed to form a compression stress layer on the surface of a glass panel in a thickness of ⅙ of the thickness of the glass by means of e.g. a physical strengthening method, as disclosed in Japanese Patent No. 2,904,067. However, it is impossible to uniformly quench a funnel or panel having a three dimensional structure and a non-uniform wall thickness distribution. Consequently, due to the non-uniform temperature distribution, a large tensile residual stress will be formed together with the compression stress, whereby the compression stress is rather limited to a level of 30 MPa at best, and it has been impossible to impart a large compression stress. Namely, when a physical strengthening method is employed, the weight reduction of the glass bulb is limited, since the compression stress which can be imparted, is relatively small.
On the other hand, it is known to reduce the weight by strengthening the surface of a glass bulb by a chemical strengthening method. This method is a method wherein certain alkali ions in glass are substituted by ions larger than the alkali ions at a temperature of not higher than the distortion point, and a compression stress layer is formed on the surface by the volume increase. For example, it can be accomplished by immersing a strontium/barium/alkali/alumina/silicate glass containing from about 5 to 8% of Na2O and from about 5 to 9% of K2O, in a molten liquid of KNO3 at about 450xc2x0 C. In the case of such chemical strengthening method, a large compression stress up to the maximum of about 500 MPa can be obtained, and it is advantageous for the weight reduction over the physical strengthening in that no unnecessary tensile stress will be formed.
Further, a cathode ray tube generates X-rays when electron beams emitted from an electron gun are bombarded to the phosphor coated on the inner surface of the panel to let the phosphor emit light to project picture images, and such X-rays are likely to give damages to human bodies if they pass through the panel and leak out of the cathode ray tube. Accordingly, in addition to the above-mentioned weight reduction, the glass forming the panel is required to have an X-ray shielding ability.
As a component to let the glass absorb X-rays, it is known to employ an oxide having a high mass absorption coefficient for X-rays, such as SrO, BaO, ZnO or ZrO2. For example, JP-A-7-206466 discloses an invention relating to a glass composition for a face of a cathode ray tube, which comprises from 58.5 to 60.5 wt % of SiO2, from 1.0 to 2.5 wt % of Al2O3, from 6 to 7.5 wt % of Na2O, from 8 to 9.5 wt % of K2O, from 8 to 9.5 wt % of SrO, from 8 to 9.5 wt % of BaO, from 1 to 2.5 wt % of ZnO, from 2.5 to 3.5 wt % of ZrO2, from 0 to 1 wt % of CaO+MgO, from 0.1 to 0.6 wt % of CeO2, from 0.3 to 0.6 wt % of TiO2 and from 0.2 to 0.5 wt % of Sb2O3.
However, such a glass composition is not suitable for a chemical strengthening method, and even if alkali ion substitution was carried out, it was not possible to form a compression stress layer having an adequate thickness required when the above-mentioned scratching by #150 emery paper is carried out.
Further, the X-ray shielding ability of the face portion of the panel exponentially changes with an increase or decrease of the product of the thickness of the face portion and the X-ray absorption coefficient of the glass forming the panel. Accordingly, when a panel employing a glass having a conventional composition, was made thin, there was a problem that the X-ray shielding ability substantially decreased.
The present invention has been made in view of the above problems, and it is an object of the present invention to solve the problem of an excessive load of the vacuum stress when the thickness is made thin, by imparting a high compression stress to the inner surface and the outer surface of the panel by a chemical strengthening method, to solve a problem of the thermal stress or the damage during the production of a cathode ray tube, such as in a step of evacuation or attaching a shadow mask and to provide a glass panel for a color cathode ray tube having an adequate X-ray shielding ability, and a cathode ray tube employing such a glass panel.
To accomplish the above object, the present invention provides a glass panel for a color cathode ray tube (hereinafter referred to as a panel) which has a layer having a compression stress of at least 70 MPa (hereinafter referred to as a compression stress layer) formed by a chemical strengthening method at least at short axis end portions and/or long axis end portions of the outer surface of a face portion and at a center portion of the inner surface of the face portion. Such a compression stress layer preferably has a thickness of at least 30 xcexcm.
Further, the glass forming the above panel preferably consists essentially of, as represented by mass percentage of oxides based on the total mass:
wherein the total content of ZnO and ZrO2 is from 2.5 to 11.0%, and the total content C1 of SrO, BaO and CaO, the content C2 of ZnO and the total content C3 of ZnO and alkaline earth metal oxides, satisfy relations of 0.6xe2x89xa6C1/C3xe2x89xa60.9 and 0.05xe2x89xa6C2/C3xe2x89xa60.4.
Further, the present invention provides a cathode ray tube employing the above glass panel for a color cathode ray tube.